The present invention relates generally to wooden frame construction materials and techniques, and more specifically to enhancing fire resistance in wooden frame members used in construction.
Since their introduction in about 1960, light timber, open-web wood trusses have become one of the most widely used engineered wood building products employed in commercial construction. According to the Wood Truss Council of America (WTCA), such trusses are lightweight, easy to install, and have nailable chords for easy attachment of roof decking and ceiling materials. Open-webbing provides great benefits to plumbers and electricians, without the need to spend time cutting holes in floor members. Less cutting reduces jobsite labor and reduces potentially critical errors that could result in compromising the structural integrity of the components. Open-web wood trusses are lighter, less expensive and can be stronger than large, single “closed web” support members.
When subject to fire damage, the weak link or cause of failure of such open-web trusses is the detachment of the metal gusset plates used to connect the framing members together. Under load, as the wood chars and the metal gusset plates heat up under fire, the teeth of the metal gusset plates lose strength and holding power. The loss of the gusset plate on the bottom chord of a truss can lead to tensile forces pulling the truss apart. The loss of a gusset plate on the top chord will cause any web members attached to the top chord to pull away. Both situations will significantly reduce the load-carrying capacity of the installed truss and may even lead to a truss collapse.
Thus, engineered building components provide adequate strength under normal loading, but under fire conditions, these truss systems can fail, leading to the collapse of roof, floors, and possibly the entire structure. Truss systems are usually hidden, and fires within truss systems can go undetected for long periods of time, resulting in loss of structural integrity prior to discovery of the fire. Structural design codes often do not factor in the decreased system integrity as the fire degrades the structural members.
A suitable approach to enhancing fire resistance to wood trusses is described in commonly-assigned U.S. Pat. No. 9,422,714 which is incorporated by reference. In that patent, the focus is on obtaining a 2-hour fire rating for a wooden truss. However, many jurisdictions have a 1-hour fire rating. In addition, there is a trend in the industry to provide relatively lightweight building panels for ceiling construction. The lighter weight panels are easier for contractors to handle and install than conventional wallboard panels. However, the newer lighter weight panels have different fire-resistant properties.
While gypsum wallboard panels have long been known to retard the progress of fire in a building, one drawback of conventional panels is that, when exposed to intense heat, the remaining moisture in the set panels calcines, causing the panels to shrink. U.S. Pat. Nos. 2,526,066 and 2,744,022 discuss the use of comminuted unexfoliated vermiculate and mineral and glass fibers in proportions sufficient to inhibit the shrinkage of gypsum panels under high temperature conditions. Both references, however, relied on a high density core to provide sufficient gypsum to act as a heat sink. They disclose the preparation of ½-inch-thick gypsum panels with a weight of, 2 to 2.3 pounds per square foot (2,000 to 2,300 pounds per thousand square feet (“lb/msf”)) and board densities of about 50 pounds per cubic foot (“pcf”) or greater.
It is noted that in the absence of water resistant additives, when immersed in water, set gypsum can absorb water up to 50% of its weight. When gypsum panels—including fire resistant gypsum panels—absorb water, they can swell, become deformed and lose strength which may degrade their fire-resistance properties. Low weight fire-resistant panels have far more air and/or water voids than conventional, heavier fire-resistant panels. These voids would be expected to increase the rate and extent of water uptake, with the expectation that such low weight fire-resistant panels would be more water absorbent than conventional heavier fire-resistant panels.
Accordingly, there is a need for open-web truss systems having enhanced fire resistance while employing the lighter weight ceiling panels.